Fiber-reinforced, ceramic composite articles have been known to the art for a number of years. In a typical construction, the articles comprise two basic components: (1) a ceramic matrix material such as a glass, a glass-ceramic, or a crystalline ceramic material; and (2) inorganic, refractory fibers which are entrained therein to provide a reinforcing phase. The patent literature contains numerous references to a wide variety of ceramic matrix materials and fiber compositions, as well as to almost limitless combinations of matrix materials and fibers. U.S. Pat. Nos. 4,314,852 and 4,615,987 provide but a small sample of the structure and production of such composites.
Fiber-reinforced composites utilizing ceramic matrix materials are typically prepared from finely-divided particles of the desired matrix materials, or a precursor for the desired matrix material. The fibers are combined with those particles and consolidated to dense composite articles via the application of heat and pressure. To assure that the fibers will be homogeneously distributed throughout the matrix material, the finely-divided powders of the matrix material are customarily first dispersed within a liquid vehicle to form a suspension or slurry. This suspension is thereafter applied to the fibers as a coating. In the case of short fibers, the fibers may be admixed into the slurry to produce a homogeneous dispersion of the fibers.
Typically, dispersions of matrix powders in liquid vehicles contain various binders, solvents, surfactants, and dispersing agents. These additives function to stabilize the dispersion and impart thereto such desirable properties as rapid drying and some flexibility in the article after drying has occurred, but before firing thereof to consolidation. A plasticizer may also be included to enhance the flexibility and, hence, the formability of the dried, but unfired, article.
Experience has demonstrated that such suspensions must be carefully formulated, handled and stored in order to be used effectively. For example, the ceramic powders are quite prone to settle out in the suspensions. Consequently, continuous mixing is frequently demanded to assure homogeneity therein. Means for stabilizing the suspensions against changes in viscosity, which changes are encountered commonly during storage or in the use thereof, may be required to avoid undesirable changes in the coating or in the fiber-impregnation characteristics of the suspension.
In summary, in the customary procedure for producing fiber-reinforced ceramic composites, it is necessary to go through an intermediate operation during which a powdered form of the ceramic is dispersed within a tow of fibers and bound in place via an organic/polymeric binder. This intermediate product, termed a "prepreg", must be capable of being fashioned into a desired shape for the final composite article. The shaped body is thereafter processed to remove ("bum out") the binder without forming significant carbonaceous residue. It is then consolidated through sintering to produce the final composite article.
U.S. Pat. No. 5,024,978 (Allaire et al.) describes a binder system which has been successfully employed in the manufacture of fiber-reinforced, ceramic composite articles. Fundamentally, that system comprises a blend of a thermoplastic organic polymeric material and a thermoplastic organic wax. The system makes it possible to uniformly impregnate the fiber tow with ceramic powder and to prevent the powder from falling away during handling and processing of the resulting prepreg. The prepreg may then be readily formed and shaped, and the binder removed therefrom without significant carbonaceous residue being developed.
Field experience with that binder system, however, has identified the following shortcomings therein encountered in the processing of the composite. These shortcomings are specifically related to the presence of a substantial amount of wax material in the prepreg.
(a) A significant amount of the thermoplastic binder is a wax component which must be removed in an initial "dewaxing" process to inhibit the flowing out of the binder and the ceramic powder during the burning out of the polymeric material. That dewaxing process must be controlled very precisely. PA1 (b) With certain matrix materials, it is necessary, or at least desirable, to remove the wax before the initial forming or shaping of the composite article. In those situations, the wax is acting only as a solvent for the polymeric materials in the binder component and, consequently, offering no benefit to the composite prepreg. PA1 (c) The wax is volatilized off during the dewaxing operation plus some wax vaporizes off during the impregnation process in which the fiber tow passes through the molten binder-powder slurry. That volatilized wax presents maintenance problems with wax buildup in the ventilation system. PA1 (d) To achieve the desired low viscosity in the binder-powder slurry to promote its impregnation into the fiber tow, the temperature of the slurry must be maintained at a relatively high temperature (customarily about 140.degree. C.). Such temperatures pose a safety concern and also contribute to vaporization of the wax.
One advantage of the wax-based thermoplastic binder system, however, is the extremely fast set which occurs upon exit from the impregnation bath. The preferred wax-based thermoplastic binder system employs a thermally-reversible gel to enhance the setting rate of the binder. This prevents powder migration and allows rapid handling of the tow. This is advantageous in processing of the tow. Also, the gel structure of the preferred wax-based thermoplastic binder system facilitates the subsequent removal of the wax components by volatilization without the occurrence of powder migration during the "dewaxing" process.
There are two fundamental alternatives to the wax-based thermoplastic binder system disclosed in U.S. Pat. No. 5,024,978 that will yield a prepreg exhibiting the desired characteristics:
The first alternative contemplates an organic solvent-based binder which will dry very rapidly after the fiber tow is impregnated with the binder-powder slurry. That has not proven to be a feasible solution to the problem. The organic solvent-vapor released upon drying creates major safety, health, and environmental issues which must be addressed. Also, even if very rapid drying solvents are employed to maintain the handling advantages of the wax-based thermoplastic binder system, it is unlikely that powder migration will be prevented.
The second alternative contemplates a water-based (aqueous) binder system. Our invention is directed to such a system.
Aqueous binders are well known to the art and have been employed commercially. The primary drawback encountered in the use of aqueous binders has been the very slow evaporation of water such that the uniform distribution of the ceramic powder cannot be readily maintained. The use of aqueous binder systems has also been limited by a need to pre-dry the impregnated fiber tow, prior to winding/forming.
The principal objective of the present invention was to develop a novel aqueous binder system for use in the manufacture of fiber-reinforced, ceramic composite articles which would avoid the problems experienced in former water-based binder systems. The rapid settling advantage of the particular aqueous binder system proposed maintains the handling and prevention of powder migration advantages of the wax-based binder system in U.S. Pat. No. 5,024,978.